Hollow core and solid core doors, used in both interior and exterior applications, normally are constructed in a similar manner. Thin wooden door skins generally are adhesively affixed to a door frame which includes two parallel wooden stiles extending along the longitudinal edges of the door and two parallel wooden rails at the bottom and top edges of the door. The door skins are spaced one from the other by the frame, defining in one case a hollow core, or in the other case, a space that is filed by a solid particleboard-type material or in some cases synthetic foam.
The various components of such door structures can be, and very often are, made from wood composite materials. Wood composite materials include substrates produced from wood particles, wood fibers, wood flakes, wood chips or wood veneers, such as hardboard, medium density fiberboard (MDF), oriented strand board (OSB), wafer board, flake board, chip board, particleboard, laminated veneer lumber (LVL) and the like. Such wood composite materials are usually prepared under heat and pressure from particles, fibers, flakes, chips or veneers, bonded together using an adhesive binder.
Hardboard panels, in particular, have found wide use as door skins. In such applications, it is common practice to coat (i.e., temper) the surface of the hardboard panel with a varnish based on a drying oil, such as linseed oil. Such oil “tempering” improves surface hardness, adds moisture/water resistance and provides a hardboard surface that tends to adhere better to the door frame and door core. Unfortunately, the conventional use of drying oils in this way requires that the panels be heat treated in a relatively high temperature oven to cure the drying oil in a reasonable amount of time. Unfortunately, the need for heat treatment complicates the door skin manufacturing procedure. Use of such a coating material in this application also is accompanied by the release of volatile organic compounds (VOCs). In particular, the oxidation and additional pyrolytic degradation of linseed oil that may accompany the oven cure results in the formation of acrolein. Acrolein is highly volatile and is considered a strong eye and respiratory irritant, particularly on mucosal membranes. Because of this behavior of linseed oil during elevated temperature cure, more stringent regulatory standards slated to be implemented in 2007 (MACT) will complicate the continued use of linseed oil in door construction and other similar applications. Thus, industry continues to look for ways to provide coating compositions that provide similar benefits yet reduce VOCs in the workplace.
The present invention provides a non-aqueous coating composition of very low volatility and low viscosity (i.e., low-VOC and generally substantially VOC-free) that can be cured in a short period of time while under an ambient temperature condition. Thus, the composition can be used, inter alia, as a tempering agent for hardboard door skins without the necessity of an elevated temperature bake oven while minimizing or eliminating VOC release.